On the contrary, alcohol is a natural occurrence as a byproduct of the yeast consuming the natural sugars in the juice. It happens in beer, mead, grains, etc. It is how bread rises, even.
That's nice, and it is true, but the phase of ethanol (EtOH) formied is only a transitional phase as the sugars pass from dissolved molecules of fructose to molecules of acetic acid (EtOOH) in vinegar, when the process is completely open to air. In such a process, the amount of ethanol at any moment is very low.
Are you not aware that in order to stop the oxidation of the sugars, the process must take place in the absence of free oxygen from the air?
One way is to place the juice in an elastic container, a new sheepskin, and seal it. Then, in presence of yeast, the sugars break down anaerobically to form EtOH and carbon dioxide (CO2). This is not a natural process. It only occurs when mankind intervenes with some kind of device to to interrupt the natural transformation to vinegar. Capisce?
A more modern way to produce wine is basically to put the juice into a container, add some manufactured sugar to drive up the alcohol percentage, add yeast, then seal with an exhaust tube in the stopper. Then connect a length of Tygon tubing to the exhaust tube, and submerge the other end of the tube in a beaker of water or vegetable oil.
This allows the excess CO2 to bubble out, without having air allowed back into the container. Then allow the process to complete. At some point, the alcohol content reaches about 12% maximum, enough to stop further enzymatic action. (Human intervention required, eh?)
Forming intoxicating beverages is a man-made process, not a natural one. It was discovered very early on, after the atmospheric conditions and yeast contamination of everything occurred after the flood. Doubtless, righteous Noah had no idea that his first batch of bottled grape juice was going to become intoxicating after the flood catastrophe.
Apparently his antediluvian culture did not know much about fermentation processes. I see little mention of bread as a principal food until Abrahams time, although God did mention it in Genesis 3. But God declares that their principal food was to be green herbs, not baked bread. Nor is there any mention that Noah formed a habit of recreational use of intoxicants subsequent to that unique post-flood experience.